Fire alarm system

ABSTRACT

A fire alarm system including a plurality of individual alarm devices connected to a two-wire electrical loop, each alarm device including a unidirectional current conducting means, a resistive impedance, and a switch adapted to move to an actuated position in response to fire detection. Also included in the loop are a bi-directional voltage source, a passive load having a predetermined value, and a resisitve impedance element also of a predetermined value, said plurality of alarm devices and the passive load forming one portion of a voltage divider, the other portion of the voltage divider being formed by the resistive impedance element. A fire alarm signal can be distinguished from all other signals generated by the system, in that under normal operation, the voltage drop across the resistive impedance element is not altered in response to the application of a first polarity voltage and then a second polarity voltage, while in response to the detection of a fire, a switch in an alarm device assumes its actuated position, whereby the voltage drop across the resistive impedance element assumes a first value in response to a first polarity voltage and a second value different from the first value in response to a second polarity voltage. In response to other abnormal conditions, such as ground leakage between the wires of the loop, the potential drop across the resistive impedance element remains the same for both polarity voltages.

Unite States Patent [191 Gysell et al.

[ 51 Aug. 27, 1974 FIRE ALARM SYSTEM [76] Inventors: Bjorn Gysell, Dianavagen 77,

' Bandhagen; Karl Axel Nilsson,

Kruthornsvagen 17, Sollentuna; Gary Nilvid, Skebokvarnsvagen 174, Stockholm, all of Sweden [22] Filed: Nov. 5, 1971 [21] Appl. No.: 196,045

[30] Foreign Application Priority Data 3,716,834 2/1973 Adams 340/409 X Primary E.raminerDonald J. Yusko Assistant E.raminerWilliam M. Wannisky Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak ABSTRACT A fire alarm system including a plurality of individual alarm devices connected to a two-wire electrical loop, each alarm device including a unidirectional current conducting means, a resistive impedance, and a switch adapted to move to an actuated position in response to fire detection. Also included in the loop are a bidirectional voltage source, a passive load having a predetermined value, and a resisitve impedance element also of a predetermined value, said plurality of alarm devices and the passive load forming one portion of a voltage divider, the other portion of the voltage divider being formed by the resistive impedance element. A fire alarm signal can be distinguished from all other signals generated by the system, in that under normal operation, the voltage drop across the resistive impedance element is not altered in response to the application of a first polarity voltage and then a second polarity voltage, while in response to the detection of a fire, a switch in an alarm device assumes its actuated position, whereby the voltage drop across the resistive impedance element assumes a first value in response to a first polarity voltage and a second value different from the first value in response to a second polarity voltage. In response to other abnormal conditions, such as ground leakage between the wires of the loop, the potential drop across the resistive impedance element remains the same for both polarity voltages.

9 Claims, 6 Drawing Figures RATENTEI] AUG 2 1 mm SHEET 1 OF 2 Fig. 7

Fig. 4

SHEET 2 OF 2 PAIENIED AUBZY 1914 FIRE ALARM SYSTEM FIELD OF THE INVENTION This invention relates to a fire alarm system having a plurality of individual alarm devices connected to a two-wire electrical loop having a definite inpedance, said individual alarm devices being adapted in actuated position to allow current to flow in only one direction and said electrical loop being connected to a voltage sourc adapted to generate a voltage having a first polarity and a second polarity.

More particularly this invention relates to a fire alarm system comprising a two-wire electrical loop terminatingwith a load having a definite impedancy, a plurality of individual alarm devices connected to said electrical loop, each of said individual alarm devices comprising a uni-directional means, an impedance and a switch adapted in case of fire to be changed over from a rest position into an actuated position, said alarm devices being adapted in their actuated state to allow current to pass in one direction only and said electrical loop being connected to a voltage source devised to generate a voltage having a first polarity and a second polarity, and a resistive impedance connected to said loop and disposed together with the load to constitute a voltage divider and a voltage measuring means connected across said resistive impedance.

THE PRIOR ART In modern fire alarm systems it is required that a central station, which in the following will be called the measuring instrument, shall receive signals which are indicative of various states, which can appear in the loop. Examples of the states to which the measuring instrument should be sensitive are the normal state, the state of fire signal, the state of short circuit, the state of interruption of the circuit and the state of ground connection. For these signals it is essential that the signal indicating fire is unique and thereby prevents a fire alarm to be emitted for example, when a connection to ground appears in the loop. In known systems which comply with this requirement the individual alarm devices become relatively expensive and include e.g., two uni-directional current conducting means to render possible conduction of current in two directions. Therefore, said systems become relatively complicated and expensive.

THE OBJECTS OF THE INVENTION One main object of the invention is thus to provide a fire alarm system which, inspite of complying with the stated requirements on distinguishing various signal states, contains one uni-directional current conducting means only in each individual device and consequently becomes cheaper and, nevertheless, more reliable in operation.

Another object of the invention is to provide such a fire alarm system that depending on the type of detector used each individual alarm device is capable of operating with either a closing contact or an opening contact connected in series to the uni-directional current conducting means in contradiction to previously known systems which comply with the above stated requirements and which can operate only with a circuit closing contact in the detector signaling the state of fire.

SUMMARY OF THE INVENTION These objects are completely realized according to the invention which substantially is based on the feature that the system is built up with a voltage divider having a passive definite load conducting current in both directions and which has the same impedance in both directions of conduction, that the alarm devices in normal operation in response to voltages of the first polarity and the second polarity do not change the normal impedance in that branch of the voltage divider which contains the load, that on actuation of the switch of at least one alarm device the impedance of said branch receives a first value at the first polarity and another value differing therefrom at the second polarity and that on ground leakage between the conductors of the loop the impedance is reduced equally in response to both polarities.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will become apparent from the following description, considered in connection with the accompanying drawings which form part of this specification and on which FIG. 1 is a more or less diagrammatic representation of a system made according to the invention and having alarm devices connected in parallel to one another between the conductors or an electrical loop,

FIG. 2 is the equivalent diagram for the system shown in FIG. 1 in its normal condition of operation,

FIG. 3 is the equivalent diagram of the system upon detection of fire,

FIG. 4 is the equivalent diagram of the system on connection to ground,

FIG. 5 is a schematical representation of a modified alarm system and FIG. 6 is a representation of a system having alarm devices connected in series to one another.

DETAILED DESCRIPTION OF THE DRAWING FIGURES Referring now to FIG. 1,- a fire alarm system constructed according to the invention is represented highly diagrammatically. The system comprises a closed electrical loop with conductors l and 2 which are connected to a resistive load R having high resistance such as 5 Kohm, for example, and which in the simplest case consists of a resistor. The closed loop is fed with voltage from a voltage source 3. Connected to the loop and a point common to the terminals of the loop is a resistor R having a low ohm value of e.g. 200 ohms, across which resistor is connected a measuring instrument 4 which measures the voltage Vm across the resistor R The closed loop thus together with the resistor R forms a voltage divider.

Connected to the closed loop are alarm devices Al, A2, A3, A4 An, and in the embodiment shown in FIG. 1, these alarm devices are connected in parallel between the conductors l and 2. Each individual alarm device, such as Al, for example, consists of a resistor R which is connected in series with a unidirectional current conducting means suchas a diode 5, and a normally open switch 6 adapted to close in case of fire.

The voltage source 3 is adapted to generate continuously or at a selectable moment a DC-voltage Vin, the polarity of which alternates with a frequency of e.g., 1 Hertz. An AC-voltage having an arbitrary frequency can also be employed, but a DC-voltage is preferred because of the field of disturbances developed around the loop when AC-voltage is supplied.

In the normal state of operation of the system shown in FIG. ll all switches are open and if the voltage source 3 supplies a continuous DC-voltage, the conductor 2 is at a positive potential relative the conductor 1. The equivalent diagram for the system in the normal state of operation is shown in FIG. 2. If the voltage source supplies Vin, the voltage Vml fed to the measuring instrument 4 will become:

Vml =Vin'R /R +R It is easily understood that on supply to the circuit of a periodically alternating DC-voltage or an AC-voltage the absolute value for Vml will remain unchanged and that only the electric sign is changed.

In case of fire whereby the switch in the alarm device A1 is closed the equivalent diagram of the system will be that of FIG. 3, and thus the resistor R is placed in parallel with the resistor R When disregarding the contact resistance in the switch 6 and the resistance in the diode 5 and when as in the earlier ease presuming that a constant DC-voltage is supplied to the conductors 1 and 2 and that the conductor 2 at a higher potential than the conductor 1, current will flow through the diode 6 and consequently the voltage measured by the measuring instrument 4 Vm2) will become:

+ Vm2 Vin 12,10, R /R R3) 2) This measured voltage Vm2 differs clearly from Vml according to the equation (1) and constitutes an alarm releasing voltage. In order to allow the signal to be identified positively as a fire indicating signal, the continuous DC-voltage is automatically by means not shown here subjected to current reversal which means that the conductor 1 becomes more positive than the conductor 2 whereby the diode 6 blocks the flow of current through resistor R and the measuring instrument 4 measures the voltage Vm2:

Vm2 Vin /R, R

By comparison of these two signals it can be established in an unmistakeable manner as will become evident from the case to be described in the following, that fire has broken out and alarm is emitted through the conductor 7 of FIG. 1.

A state similar to fire is a ground connection in the loop and this state must not result in the generation of a fire alarm. A connection to ground of the conductors l and 2 means that a resistance is connected in parallel to R This resistance is illustrated in the equivalent diagram of FIG. 4 by the resistor R Since no switch 6 has been closed, no diode 6 has been switched in and the voltage Vm3 measured by the measuring instrument 4 will be independent of the polarity of the voltage from the voltage source 3 which means that -Vm3 will become equal to Vm3.

When R R the fire indicating signals according to the equations (2) and (3) will be easily distinguishable from both signals according to the equation (4) and a fire alarm will not be produced.

In the case of a simple grounding of one of the conductors, the system will not be actuated if the voltage source 3 and the measuring instrument 4 are insulated from ground. In case that the grounded system according to FIG. 1 becomes one or the other conductor connected to ground, signals will be generated which are easy to distinguish from other possible signals and therefore no fire alarm will be given.

On short circuiting of the conductors 1 and 2 in the diagram of FIG. 1 and when disregarding the resistance of the conductors, the voltage source 3 will be connected directly across the resistor R and therefore the measuring instrument 4 will be supplied with the whole voltage Vin irrespective of the polarity thereof. This signals indicating a fault is also easily distinguishable from the fire indicating signals and therefore no fire alarm will be started.

It is easy to show that other possible faulty conditions in the electrical loop such as, for instance, interruption in a conductor and ageing in a contact will create measured signals which significantly distinguish from other signals and thus do not cause a fire alarm to be given.

In its simplest realization, the measuring instrument 4 may consist of a voltage meter adapted to transfer to the alarm emitting conductor signals according to the equations (2) and (3), or an analogue-digital converter or some other suitable device.

The voltage source 3 may be a DC-voltage source devised continuously to feed a DC-voltage to the loop and equipped with a polarity inverter which becomes operative when an abnormal voltage appears across the resistor R or some other resistive means and which periodically reverses the current direction of the DC- voltage supplied to the conductors 1 and 2. If desired, the current direction may be reversed all the time during the normal conditions of operation of the system. As mentioned above, the voltage source 3 may also be adapted to generate an AC-voltage having an arbitrary frequency.

The shown switch 6 may be a mechanical contact which in case of fire is closed by means not shown here, or a transistor or a controlled silicon rectifier which receives a control signal from a smoke detector or the like or some other conventional detecting device. In case a controlled rectifier is used said rectifier also replaces the separately shown rectifier 5 as is illustrated in FIG. 5 which shows a modification of the alarm device A].

As will be seen from FIG. 5, the silicon controlled rectifier 5 replaces both the diode 5 and the switch 6 of FIG. 1. The control electrode 7 of the silicon rectifier is normally dead, but connected to a detector not shown here such as e.g., a flue gas detector which as soon as flue gas or smoke appears generates a signal to the control electrode 7 whereby the silicon rectifier is made capable of conducting current in one direction, i.e., in the illustrated case when the conductor 2 is at a higher potential than the conductor 1.

FIG. 6 shows a system according to the invention intended for fire alarm systems with a normally closed switch 6', which is opened in case of fire and consists e.g., of a simple fuse. In this case the switch 6' is connected in parallel with the diode 5 and the resistor R; in each alarm device, e.g., the alarm device Al. The alarm devices Al A'n are in this case connected in series with one another in the simple loop consisting of the conductor 1', the load resistance R, and the conductor 2. The voltage source 3 which is of the same kind as the source described hereinbefore is with its one pole connected to the conductor 1' and with its other pole to one end of the resistor R the other end of which is connected to the conductor 2'. The measuring instrument 4' which is of the same kind as the measuring instrument 4 described above is connected over the resistor R' and provided with a conductor 7' for the generation of fire alarm signal.

With the the diodes position as shown and with a constant DC-voltage from the voltage source 3', a positive potential is suitably connected to the conductor 2' whereby under the normal conditions of operation the voltage measured by the measuring 4' on a supplied voltage Vin from the voltage source 3 will be:

If the switch 6' breaks, the measuring instrument 4 will under the conditions set out above measure the voltage This deviation from the normal value means that the voltage source is subjected to current reversion as earlier described, and hereby the loop will be closed over the diode and the measuring instrument 4' will measure the voltage whereby it is confirmed that the case of fire actually is at hand.

It can be shown that the signals 'Vm5 and Vm6 differ from the signals generated by other possible conditions in the circuit such as short circuiting, connection to ground or conductor breaks.

While several embodiments of. the invention have been shown and described, it is to be understood that this is for purpose of illustration only and that the invention is not to be limited thereby, but its scope is to be determined by the appended claims.

What is claimed is:

l. A fire alarm system comprising a pair of common conductors (1, 2; 1', 2) terminating with a load resistor (R,; R',) having a predetermined resistance, a plu-,

rality of individual alarm devices connected to the electrical loop constituted by said conductors and said load resistor, each of said individual alarm devices comprising a single unidirectional current conducting means (5; 5') in series with a resistive element (R R;,) and a switch (6; 6'; 5a) adapted in case of fire to be changed over from a rest position into an actuated position, said switches being arranged in their rest positions to prevent current from flowing through the respective unidirectional current conducting means and the series connected resistive element, and in their actuated positions to allow current to flow in one direction only through said unidirectional current conducting means and the series connected resistive element, and a measuring resistor R R' connected with its one end to one of the conductors and with its other end to one terminal of a voltage source (3; 3'), the other terminal of said voltage source being connected to the input end of the other conductor, and said voltage source being devised to generate a voltage having a first and a second polarity, the measuring resistor being disposed together with the load resistor to constitute a resistive voltage divider, and a voltage measuring means (4; 4') connected in parallel with said measuring resistor, wherein the load resistor and the measuring resistor are passive and adapted to allow current to flow in both directions and which have the same resistance in both directions of conduction, so that the alarm devices in normal, non-actuated position of their respective switches do not, in response to voltages of the first polarity' and the second polarity, change the normal impedance in that branch of the voltage divider, which contains the load resistor, that on actuation of the switch of at least one alarm device the impedance of said branch receives a first value at the first polarity and another value differing therefrom at the second polarity and that on short circuit or leakage between the conductors the impedance is reduced equally in response to both polarities.

2. The fire alarm system as claimed in claim 1, wherein each alarm device includes a resistor (R which is connected in series to a uni-directional current conducting element (5) and to a normally open switch (6) said alarm devices being connected in parallel with the load.

3. The fire alarm system as claimed in claim 1, wherein each alarm device includes a resistor (R;,) serially connected to a uni-directional current conducting element (5') and a normally closed switch (6) connected in parallel to said resistor and said element, said alarm devices being connected in series with one another and the load.

4. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting elements is a diode (5, 5').

5. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting element (5; 5') and the associated switch (6; 6) are constituted by a single controlled silicon rectifier (5a) the control electrode (7) of said controlled rectifier receiving an actuating signal in case of fire.

6. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting element and the associated switch consists of a transistor disposed in the case of fire to receive actuating signal from a detector.

7. The fire alarm system as claimed in claim 3, wherein said uni-directional current conducting element is a diode (5; 5').

8. The fire alarm system as claimed in claim 3 wherein said uni-directional current conducting element (5, 5') and the associated switch (6, 6') are constituted by a single controlled silicon rectifier (5a) the control electrode (7) of said controlled rectifier being diislposed in the case of fire to receive an actuating sign 9. The fire alarm system as claimed in claim 3, wherein said uni-directional current conducting element and the associated switch consists of a transistor disposed in the case of fire to receive actuating signal vfrom a detector. 

1. A fire alarm system comprising a pair of common conductors (1, 2; 1'', 2'') terminating with a load resistor (R1; R''1) having a predetermined resistance, a plurality of individual alarm devices connected to the electrical loop constituted by said conductors and said load resistor, each of said individual alarm devices comprising a single unidirectional current conducting means (5; 5'') in series with a resistive element (R3; R''3) and a switch (6; 6''; 5a) adapted in case of fire to be changed over from a rest position into an actuated position, said switches being arranged in their rest positions to prevent current from flowing through the respective unidirectional current conducting means and the series connected resistive element, and in their actuated positions to allow current to flow in one direction only through said unidirectional current conducting means and the series connected resistive element, and a measuring resistor R2; R''2) connected with its one end to one of the conductors and with its other end to one terminal of a voltage source (3; 3''), the other terminal of said voltage source being connected to the input end of the other conductor, and said voltage source being devised to generate a voltage having a first and a second polarity, the measuring resistor being disposed together with the load resistor to constitute a resistive voltage divider, and a voltage measuring means (4; 4'') connected in parallel with said measuring resistor, wherein the load resistor and the measuring resistor are passive and adapted to allow current to flow in both directions and which have the same resistance in both directions of conduction, so that the alarm devices in normal, non-actuated position of their respective switches do not, in response to voltages of the first polarity and the second polarity, change the normal impedance in that branch of the voltage divider, which contains the load resistor, that on actuation of the switch of at least one alarm device the impedance of said branch receives a first value at the first polarity and another value differing therefrom at the second polaRity and that on short circuit or leakage between the conductors the impedance is reduced equally in response to both polarities.
 2. The fire alarm system as claimed in claim 1, wherein each alarm device includes a resistor (R3) which is connected in series to a uni-directional current conducting element (5) and to a normally open switch (6) said alarm devices being connected in parallel with the load.
 3. The fire alarm system as claimed in claim 1, wherein each alarm device includes a resistor (R''3) serially connected to a uni-directional current conducting element (5'') and a normally closed switch (6'') connected in parallel to said resistor and said element, said alarm devices being connected in series with one another and the load.
 4. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting elements is a diode (5, 5'').
 5. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting element (5; 5'') and the associated switch (6; 6'') are constituted by a single controlled silicon rectifier (5a) the control electrode (7) of said controlled rectifier receiving an actuating signal in case of fire.
 6. The fire alarm system as claimed in claim 2, wherein said uni-directional current conducting element and the associated switch consists of a transistor disposed in the case of fire to receive actuating signal from a detector.
 7. The fire alarm system as claimed in claim 3, wherein said uni-directional current conducting element is a diode (5; 5'').
 8. The fire alarm system as claimed in claim 3 wherein said uni-directional current conducting element (5, 5'') and the associated switch (6, 6'') are constituted by a single controlled silicon rectifier (5a) the control electrode (7) of said controlled rectifier being disposed in the case of fire to receive an actuating signal.
 9. The fire alarm system as claimed in claim 3, wherein said uni-directional current conducting element and the associated switch consists of a transistor disposed in the case of fire to receive actuating signal from a detector. 